JP7260731B2 - High purity steel and its refining method - Google Patents

Description

TECHNICAL FIELD The present invention relates to steel for high-cleanliness steel sheets used for shipbuilding materials, line pipes, thick steel sheets, oil well pipes, etc., and a method for producing the same. More specifically, the composition of non-metallic inclusions in steel is precisely controlled within a specific range. The present invention relates to a steel having high cleanliness by simultaneously reducing multiple types of harmful inclusions, and a method for producing the same.

For the purpose of improving corrosion resistance, toughness, workability, etc., many techniques have been developed to reduce or render harmless non-metallic inclusions (hereinafter referred to as inclusions) in steel. In particular, reduction of inclusions is likely to be subject to industrial restrictions such as extension of treatment time, so many techniques have been developed for morphology control to render inclusions harmless. For example, a technique for suppressing spheroidization of inclusions and MnS by adding Ca to molten steel to turn the inclusions into CaO-containing oxysulfides is well known.

Furthermore, in recent years, a number of techniques have been developed to control inclusions by using lanthanoids such as La, Ce, and Nd or Ca and lanthanoids in combination to improve both castability and steel performance.

For example, Patent Document 1 discloses a technique for improving surface detergency and ridging resistance by containing 0.001 to 0.05% of lanthanoids, while Patent Document 2 discloses a technique of containing 0.0001 to 0.03% of lanthanoids. Techniques for improving castability by controlling the Al concentration and the lanthanide concentration in the steel to appropriate conditions when producing stainless steel have been disclosed.

Further, in Patent Document 3, the lanthanoid concentration in the steel is set to 0.001 to 0.0044%, and the lanthanoid oxide concentration in the inclusions is controlled within an appropriate range to achieve excellent castability, surface cleanliness, and internal quality. As a technique for obtaining flakes, Patent Document 4 discloses a technique for improving surface cleanliness by appropriately controlling the concentrations of CaO and lanthanide oxides in inclusions.

Furthermore, Patent Literature 5 discloses a technique for manufacturing a better steel material by controlling the concentration of Ti oxides in addition to Ca oxides and lanthanide oxides in inclusions.
As described above, many techniques utilizing lanthanoids or lanthanoids and Ca have been developed.

By the way, until now, many of the performance requirements for steel materials were for single characteristics, such as corrosion resistance and weldability. . For example, in order to improve resistance to hydrogen-induced cracking, techniques such as adding Ca to molten steel to change inclusions from MnS to CaS are well known in order to suppress MnS, which is the origin of cracking.

However, in recent years, steel materials are required to have multiple performances, and it is becoming common to be required to simultaneously satisfy, for example, corrosion resistance, low-temperature toughness, and weldability. In order to meet such demands, it is necessary to take countermeasures against multiple types of inclusions, and the conventional techniques for the purpose of dealing with limited inclusions have not been able to sufficiently meet these requirements. There is also a method of applying a plurality of types of conventional techniques to deal with a plurality of types of inclusions, but industrial production has been difficult for reasons such as the extension of processing time.

Furthermore, the following method is also known as a method for dealing with multiple types of inclusions at the same time. Harmful inclusions include Ti-based carbonitrides such as MnS and TiN, coarse oxysulfides such as Ca and lanthanides, and coarse alumina clusters. N is common. For example, if the harmful inclusions are to be suppressed at the same time, the constituent elements S, O, and N should be reduced at the same time. It is necessary to carry out all the processes for removing substances, and furthermore, to improve the performance of steel materials, it is necessary to thoroughly implement each process. For this reason, the refining cost, especially the secondary refining cost, has increased significantly, and there has been the problem that inexpensive production is difficult.

In addition, as mentioned above, lanthanoids are effective in making inclusions harmless, but there was the problem of nozzle clogging during continuous casting. Although these techniques have suppressed nozzle clogging, they have not been able to prevent inclusions from adhering to the inside of the nozzle even if they do not lead to clogging. Nozzle clogging leads to the suspension of casting, and adhesion inside the nozzle has not been considered a major issue because it is possible to cast. When inclusions adhere to the nozzle, the uniformity of the nozzle cross section is lost, resulting in non-uniform molten steel flow in the continuous casting mold. Inhomogeneous flow induces phenomena such as deterioration of flotation and separation of inclusions in the mold, promotion of aggregation and enlargement of inclusions, and accumulation of inclusions at specific positions of the cast slab. Therefore, there is an increasing need to suppress not only nozzle clogging but also adhesion inside the nozzle.

JP-A-2001-279388 JP-A-2001-192723 Japanese Patent Application Laid-Open No. 2006-97110 JP-A-2000-8138 JP-A-11-343516

In view of the above problems, the present invention simultaneously significantly reduces coarse inclusions, MnS, and Ti-based carbonitrides by controlling the concentrations of Ca and lanthanoid oxysulfides and Al oxides in inclusions within a specific range. At the same time, it is an object of the present invention to provide a steel and a method for producing the same, which can be industrially easily produced with remarkably improved casting stability during continuous casting.

The present inventors have made intensive studies to achieve the above object, and as a result, have found that the cleanliness of steel is improved by controlling the composition range of inclusions within a predetermined range.
The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In mass %, C: 0.002% or more and 0.400% or less, Mn: 0.1% or more and 2.0% or less, Si: 0.001% or more and 1.000% or less, S: 0.001% or more and 1.000% or less. 001% or more and 0.005% or less, Al: 0.005% or more and 1.000% or less, O: 0.005% or less, Ca: 0.0005% or more and 0.0035% or less, Ti: 0.01% or more Steel containing 0.30% or less and one or more of lanthanoids at a total concentration of 0.0003% or more and 0.0020% or less, the balance being Fe and inevitable impurities, The composition of individual nonmetallic inclusions of 3 μm or more and 10 μm or less containing one or more of oxysulfides, Ca oxysulfides, Al oxides, and two or more of these is the total of the above compounds is 100, the Ca oxysulfide concentration is 30% or more and 90% or less by mass, and the mass concentration ratio of the lanthanide oxysulfide and the Al oxide (lanthanoid oxysulfide concentration) / (Al oxide concentration) is 1.0 or more and 2.4 or less, and the number of nonmetallic inclusions satisfying the composition is 95% or more of the total number of nonmetallic inclusions. .

(2) C: 0.002% or more and 0.400% or less, Mn: 0.1% or more and 2.0% or less, Si: 0.001% or more and 1.000% or less, S: 0.001% or more and 1.000% or less; 001% or more and 0.005% or less, Al: 0.005% or more and 1.000% or less, N: 0.007% or less, O: 0.005% or less, Ca: 0.0005% or more and 0.0035% or less , Ti: 0.01% or more and 0.30% or less, one or more of the lanthanoids with a total concentration of 0.0003% or more and 0.0020% or less , and the balance being Fe and unavoidable Composition of individual non-metallic inclusions of 3 µm or more and 10 µm or less containing impurities and containing one or more lanthanoid oxysulfides, Ca oxysulfides, Al oxides, and two or more of these However, the concentration of Ca oxysulfide is 30% or more and 90% or less in mass% when the total of the above compounds is 100, and the mass concentration ratio of lanthanide oxysulfide and Al oxide (lanthanoid oxysulfide concentration)/(Al oxide concentration) is 1.0 or more and 2.4 or less, and the number of nonmetallic inclusions satisfying the above composition is 95% or more of the total number of nonmetallic inclusions. wherein 35% to 50% of the total Ca addition amount is added during refining, and after 1 minute or more and 3 minutes or less, one selected from the group consisting of lanthanoid oxides Alternatively , the method for refining the high-cleanliness steel is characterized by adding a flux obtained by mixing two or more kinds and the remaining Ca as Ca or a Ca alloy.

INDUSTRIAL APPLICABILITY According to the present invention, highly clean high-performance steel can be produced efficiently and stably.

FIG. 4 is a diagram showing the relationship between the composition of inclusions, the state of inclusions, and nozzle adhesion. FIG. 4 is a diagram showing the relationship between the amount of preceding Ca, the addition interval, and inclusion control accuracy;

The present invention will be described in detail below.
First, steel components other than iron to be treated in the present invention were specified for the following reasons. In this specification, "%" in steel composition and lanthanide concentration (details will be described later) means "% by mass" unless otherwise specified.

C: C not only acts as a deoxidizing element under reduced pressure, but also affects the activities of S and N. Therefore, if the C content is less than 0.002%, the oxygen-lowering effect becomes unstable, and if the C content exceeds 0.400%, the activity of S changes greatly and the reaction mechanism changes. Therefore, C is set to 0.002% or more and 0.400% or less.

Mn: Mn is also a deoxidizing element and is an essential element because it improves various steel properties. Therefore, if the content is less than 0.1%, deoxidation becomes unstable, and if the content exceeds 2.0%, the activity of S decreases, making desulfurization difficult. Therefore, the Mn concentration is set to 0.1% or more and 2.0% or less.

Si: Si is also an essential element for stable deoxidation like Mn. , it becomes difficult to control the inclusion composition intended by the present invention. Therefore, Si should be 1.000% or less.

Al: Al is an element having the strongest deoxidizing power, and is essential for realizing low O, low S and low N. 0.005% or more is necessary to obtain this deoxidizing effect. On the other hand, when the content exceeds 1.000%, the concentration of dissolved oxygen increases again, making it difficult to achieve low O. Therefore, the content must be 1.000% or less.

S: S is an element to be removed, but if it exceeds 0.005%, a large amount of MnS is generated in addition to sulfides such as lanthanides and Ca, resulting in a decrease in inclusion control precision. On the other hand, if it is less than 0.001%, the amount of the desulfurizing agent used is greatly increased, resulting in an increase in cost. Therefore, in the present invention, molten steel with a content of 0.001% or more and 0.005% or less is treated.

O: O is an element to be removed, but if Si, Al and Mn are within the above concentration range, a large amount of nonmetallic inclusions (hereinafter referred to as " "Inclusions") will be present in the molten steel. Therefore, the O concentration was set to 0.005% or less.

Ca: Ca is an effective element for deoxidizing and desulfurizing, and at the same time it is effective for inclusion morphology control. If the Ca concentration is less than 0.0005%, deoxidation will be insufficient, and the amount of lanthanoids required for deoxidation will increase. When the Ca concentration exceeds 0.0035% and increases, the formation of CaS inclusions becomes active, suppressing the formation of the lanthanoid sulfides intended by the present invention. Therefore, in the present invention, Ca is set to 0.0005% or more and 0.0035% or less.

Ti: Ti is an element effective for improving strength, ductility and workability by precipitation strengthening. 0.01% or more is necessary to obtain these effects. On the other hand, Ti reacts with the mold powder during continuous casting to alter the quality of the mold powder, so the Ti content was made 0.30% or less.

Lanthanoids: These lanthanoids are constituent elements of inclusions for obtaining the clean steel targeted by the present invention, and contain one or more of them. If the total concentration of these substances is less than 0.0003%, lanthanide compounds cannot be formed in inclusions. On the other hand, if the content exceeds 0.002%, the activity of S is reduced, and it is predicted that the reaction rate between S and inclusions is lowered. Therefore, in the present invention, the total concentration of one or more lanthanoids is set to 0.0003% or more and 0.002% or less. It is well known that the lanthanoid elements with element numbers 57 to 71 have very similar chemical properties, but the elements with element numbers 57 to 61 are industrially used, and are used inexpensively and in large quantities. Since La , Ce, and Nd can be used, it is desirable to use these elements.

In addition, for the purpose of ensuring strength and corrosion resistance, the present invention may contain components such as Cu, Ni, Nb, V, Cr, and Mo in the range of 0.005% or more and 3% or less as necessary. does not affect the effects of

Next, the reason for limiting the inclusion composition range will be explained.
In the steel composition system described above, coarse inclusions of 10 μm or more are formed by composite inclusions of alumina, Ca-based oxysulfides composed of CaS and CaO, and lanthanoid-based oxysulfides composed of Ce ₂ O ₃ and CeS. There are three types of TiN-based carbonitrides consisting of MnS, TiN, TiC, NbC, and the like.

From the formation mechanism of inclusions, it is estimated that composite inclusions consisting of oxysulfides are formed at high temperatures from the molten steel stage to the initial stage of solidification. TiN-based carbonitrides are produced. In other words, the state of the composite inclusions that are formed first is important for suppressing the formation of MnS and TiN-based carbonitrides.

In addition, the formation of sulfides in the composite inclusions reduces the S concentration in the steel, which suppresses the formation of MnS, and the precipitation of TiN-based carbonitrides on the surface of the composite inclusions inhibits the formation of TiN-based carbonitride inclusions. Since it is suppressed, the composition of the composite inclusion strongly affects the state of the inclusion as a whole.

Furthermore, since the adhesion inside the nozzle during continuous casting can be considered as the adhesion of composite inclusions, the state of adhesion is also strongly influenced by the composition of the composite inclusions.

Based on the above considerations, the relationship between the composition of composite inclusions, the state of other inclusions, and the state of adhesion of inclusions in the nozzle was earnestly investigated by the method described below.

300 kg of molten steel was melted in a high-frequency induction vacuum melting furnace, and the composition was controlled within the ranges described above. After that, molten steel was cast into a mold through an intermediate container having a nozzle (30 mmφ×250 mmL) simulating an immersion nozzle. A sample was cut from the obtained steel ingot, and inclusions were observed using SEM-EDS with a field of view of 20×20 mm. Inclusions of 3 μm or more were taken as objects of observation, and inclusions larger than 10 μm were taken as coarse inclusions. Inclusions are composed of a plurality of elements, and _the composition of inclusions was calculated with the sum of _Al2O3 , _LAN2O3 , _LANS , CaO, and CaS as ₁₀₀ , and _Al2O3 and _{LAN2O 3} and the sum of CaO and CaS. In this study, metal Ce, which is widely used industrially as a lanthanoid, a mixture of metal La and metal Ce, or a mixture of metal La, metal Nd, and metal Ce was used. LAN indicates a lanthanoid element such as La, Ce, Nd. In addition, the number and size of carbonitride inclusions containing MnS, Ti, Nb, C, N, etc. as main components were measured as TiN. As for the adhesion to the nozzle, the nozzle was collected, and the longitudinal section was observed to observe whether inclusions adhered to the inner wall of the nozzle.

The observation results are shown in FIG. The unsatisfactory size shown in the figure means that the size of the composite inclusion exceeded 10 μm. The state of inclusions in the steel changes depending on the composition of inclusions. Coarse inclusions, TiN, and MnS are all absent, and no adhesion to the nozzle is shown. was the result. In addition, TiN carbonitride, MnS, and inclusions larger than 10 μm were judged not to exist when they could not be confirmed within the observation field of view. In this case, it was determined that there was adhesion to the nozzle, and indicated by □ in FIG. 1 . When TiN was observed in the observation field, it was indicated as unsatisfactory TiN, and when MnS was confirmed, it was indicated as unsatisfactory MnS. Moreover, no difference was observed in the effects of additives such as metal Ce, a mixture of metal La and metal Ce, or a mixture of metal La, metal Nd and metal Ce.

Therefore, the composition of nonmetallic inclusions composed of lanthanide oxysulfides, Ca oxysulfides, and Al oxides has a Ca oxysulfide concentration of 30% or more and 90% or less by mass and lanthanoid oxysulfides. By controlling the composition of inclusions within a narrow composition range in which the mass concentration ratio of oxides of metal and Al (concentration of lanthanoid oxysulfide)/(concentration of Al oxide) is 1.0 or more and 2.4 or less, conventionally difficult It was found that it is possible to suppress a plurality of harmful inclusions, which was the problem, and at the same time suppress performance deterioration due to casting instability. This composition is hereinafter referred to as the preferred range.

Next, a method for controlling the composition of inclusions within this narrow preferred range was investigated. In the present invention, Ca and lanthanoids are used in combination, and as a general addition method, a method of adding metal or alloy Ca and lanthanoids at the same time, adding lanthanoids after adding Ca, and adding Ca after adding lanthanoids is known.

Therefore, these methods were used to investigate the control accuracy to the preferred range. The experimental method was the same as the method described above, and the method of adding Ca and lanthanoids was simultaneous addition of CaSi alloy and lanthanide (misch metal), addition of lanthanide 2 minutes after addition of CaSi alloy, and addition of CaSi alloy 2 minutes after addition of lanthanide. There are 3 types. The CaSi alloy has a Ca content of 34%. The composition of inclusions was investigated by varying the addition amount of CaSi alloy and the addition amount of lanthanoids in the range of 0.01 to 0.1 kg/ton of molten steel, and accuracy = (Fig. The number of inclusions falling within the preferred range of 1/total number of inclusions x 100) was defined and used as an index. The total number of inclusions of Ca, Al, and lanthanoids refers to Ca-based oxysulfide inclusions and Al oxides of 3 µm or more that were confirmed when inclusions were observed using SEM-EDS in a field of view of 20 mm × 20 mm. The total number of inclusions, lanthanoid oxysulfide inclusions, and inclusions containing two or more of these inclusions. It is the total number of inclusions having a component range of oxide (Al ₂ O ₃ ) and lanthanoid oxysulfide (LAN ₂ O ₃ +LANS) of 3 μm or more and 10 μm or less satisfying the preferred range in FIG. In addition, since various product defects can be sufficiently suppressed if inclusions of 95% or more are in a suitable range, the inclusion control accuracy defined above is aimed at 95% or more in the present invention. . As a result, the inclusion control accuracy was 20 to 40% by the general addition method of simultaneous addition of metallic Ca or Ca alloy a and lanthanoid, addition of Ca after addition of lanthanide, and addition of Ca after addition of lanthanide. In other words, when metal or alloy Ca and lanthanoids are used, one of them becomes dominant due to strong activity, and it is difficult to control the composition to a preferred range, which is an intermediate composition range between Ca compounds and lanthanoid compounds.

In order to further enhance the dominance of Ca, it is appropriate to add only Ca and temporarily create a state of deoxidation by Ca alone before adding the lanthanide oxide. However, if this state is too strong, the effect of the lanthanoid disappears, and if it is too weak, it becomes the same as simultaneous addition. That is, the amount of Ca to be added first and the interval between the addition of the mixture of Ca and lanthanide oxide to be added subsequently are important. Based on the above studies, molten steel experiments were conducted to investigate the effect of the amount of Ca added first and the interval between the addition of the mixture of Ca and lanthanide oxides subsequently added on the accuracy of inclusion control.

The results are shown in FIG. ♦, which is the result of simultaneous addition of Ca and lanthanoid oxides, indicates an inclusion control accuracy of 48.1%, which is higher than the 20 to 40% inclusions obtained by the above-mentioned conventionally known method even with the combination of Ca and lanthanoids. Inclusion control accuracy of 70% or more was obtained regardless of the other pre-added Ca content and time interval, but the amount of Ca added at the beginning, which is the pre-added Ca content, was the total addition amount. It can be seen that the target inclusion control accuracy can be remarkably improved to 95% or more only when the amount is 35% or more and 50% or less and the addition interval is 1 minute or more and 3 minutes or less.

From the above, 35% or more and 50% or less of the total Ca addition amount is added to the molten steel, and after 1 minute or more and 3 minutes or less, one selected from the group consisting of La ₂ O ₃ , CeO ₂ , Nd ₂ O ₃ or It has been found that the composition of inclusions can be stably controlled within a suitable range by adding a flux obtained by mixing two or more types and the remaining Ca content as Ca or a Ca alloy.
In the present invention, addition of 35% or more to 50% or less of the total amount of Ca added as described above does not matter at any time as long as it is in the state of molten steel, and after 1 minute or more and 3 minutes or less, lanthanoid oxide A flux obtained by mixing one or more selected from the group consisting of Ca and the remaining Ca as Ca or a Ca alloy may be added.
After carrying out the present invention by the above method, the ladle is transferred to a continuous casting apparatus for casting.

250 tons of molten iron is charged into a top and bottom blowing converter, and decarburization blowing is performed until the C content in the molten iron reaches 0.03 to 0.2%. At the time of tapping, various deoxidizers and alloys were added to control the concentrations of C, Si, Mn, P and S in the molten steel within the above-described ranges. Further, Al and CaO were added during tapping to adjust the CaO/Al ₂ O ₃ weight ratio in the slag to 2 to 2.5 and the total concentration of FeO and MnO in the slag to 3% or less. The Al concentration in the molten steel at this time was 0.007% to 0.01%.

After that, the ladle was transferred to RH and RH treatment was started. In the RH, the temperature was adjusted first, followed by the molten steel composition adjustment (alloy addition). In this test, the steel grades were C: 0.01-0.035%, Mn: 1.2-1.3%, Si: 0.2-0.4%, S: 0.003%, Al: 0 .03-0.07%, O: 0.002-0.003%, Ti: 0.01-0.2% or C: 0.002-0.003%, Mn: 0.2-0. %, Si: 0.2-0.3%, Al: 0.007-0.015%, O: 0.0025-0.004%, Ti: 0.05-0.07% and C: 0.07%. 1-0.3%, Mn: 0.7-1.3%, Si: 1.2-2.3%, Al: 0.3-0.5%, O: 0.00075-0.0015% , Ti; After that , dehydrogenation treatment was performed with the pressure in the vacuum chamber set to 100 to 400 Pa, and the RH treatment was completed.

The ladle was moved to a refining apparatus capable of blowing an inert gas, and the molten steel was immersed with an immersion lance from the surface of the molten steel to 4/5 of the depth of the molten steel, and Ar gas was blown at 3 Nm ³ /min.
Next, a CaSi alloy was blown into the molten steel at a rate of 0.03 kg/(min·ton) at a rate of 0.03 to 0.1 kg/ton as pure Ca, and only Ar gas was blown for 2 minutes after the blowing. After that, the CaSi alloy was mixed with 0.13 kg/ton of pure Ca and cerium oxide, a mixture of cerium oxide and lanthanum oxide, or oxides of misch metal (25% La oxide, 55% Ce oxide, 15% Nd oxide, A mixture of 0.15 kg/ton of 5% Pr oxide) was injected into the molten steel. The total blowing speed was 0.06 kg/(min·ton). The reason why the amount of CaSi added initially is changed is to generate various inclusions by intentionally changing the amount of CaSi added.

After adding CaSi for the second time, only Ar gas was blown for 3 minutes to end the treatment, and a slab was cast by a continuous casting machine. The slab is 200 mm thick and 1300 mm wide, and the casting speed is 1.3 m/min.
The total lanthanide concentration of La, Ce, Nd and Pr in the steel was 0.005-0.0018%, and the Ca concentration was 0.0006%-0.0031%.

Samples of 20 x 20 x 10 mm were taken from 1/2, 1/4, and 3/4 width positions in the width direction of the obtained slab and from 1/2 and 1/4 thickness positions from the surface in the thickness direction, The skin side of the 20×20 mm surface was polished, and inclusions were measured by SEM-EPMA. Inclusions of 3 μm or more were taken as objects of observation, and inclusions larger than 10 μm were taken as coarse inclusions. Inclusions are composed of a plurality of elements, and the composition of inclusions is calculated with the sum of Al ₂ O ₃ , RE ₂ O ₃ , RES, CaO, and CaS as 100, and Al ₂ O ₃ and LAN ₂ O The ternary system of the sum of ₃ LANS and the sum of CaO and CaS was arranged. Note that LAN indicates a lanthanoid element such as La, Ce, Nd, and Pr, and the totals of compounds of these elements are LAN ₂ O ₃ and LANS. In addition, the number and size of carbonitride inclusions containing MnS, Ti, Nb, C, N, etc. as main components were measured as TiN.

As for the adhesion to the nozzle, the nozzle was collected, and the longitudinal section was observed to observe whether inclusions adhered to the inner wall of the nozzle.
Tables 1 and 2 show inclusion observation results. It can be seen that only when the composition of inclusions satisfies the first aspect of the present invention, coarse inclusions, MnS, and TiN-based carbonitrides can be simultaneously suppressed, and adhesion to nozzles can also be suppressed.

Next, the effect of treatment method was evaluated. The total amount of CaSi added is 0.18 kg/ton in terms of pure Ca, 0.15 kg/ton of CeO ₂ , and 0.12 kg/ton of metal Ce when lanthanoids are used. additions were made. The treatment is the ladle refining equipment conditions described above after the end of RH.
The range of concentrations of lanthanides and Ca in the steel was the same as described above.

Inclusions were investigated in the same manner as described above, and accuracy was obtained. Table 3 shows the results. It can be seen that the accuracy is extremely high when according to claim 2 of the present invention. Moreover, these effects were confirmed in any of the three types of steel described above, and it was confirmed that the effects of the present invention can be obtained within the range of steel composition specified in the present invention.
As described above, according to the present invention, the cleanliness of molten steel can be stably improved.

Claims (2)

% by mass, C: 0.002% or more and 0.400% or less, Mn: 0.1% or more and 2.0% or less, Si: 0.001% or more and 1.000% or less, S: 0.001% or more 0.005% or less, Al: 0.005% or more and 1.000% or less, O: 0.005% or less, Ca: 0.0005% or more and 0.0035% or less, Ti: 0.01% or more and 0.30% % or less, the total concentration of one or more lanthanoids is 0.0003% or more and 0.0020% or less, and the balance is Fe and inevitable impurities,
The composition of lanthanoid oxysulfides consisting of one or more lanthanoids, Ca oxysulfides, Al oxides, and individual nonmetallic inclusions of 3 μm or more and 10 μm or less containing two or more of these is the above compound. The concentration of Ca oxysulfide is 30% or more and 90% or less by mass in terms of concentration with the total being 100, and the mass concentration ratio of lanthanide oxysulfide and Al oxide (lanthanoid oxysulfide concentration) / (Al oxide concentration) is 1.0 or more and 2.4 or less, and the number of nonmetallic inclusions satisfying the composition is 95% or more of the total number of nonmetallic inclusions. . % by mass, C: 0.002% or more and 0.400% or less, Mn: 0.1% or more and 2.0% or less, Si: 0.001% or more and 1.000% or less, S: 0.001% or more 0.005% or less, Al: 0.005% or more and 1.000% or less, N: 0.007% or less, O: 0.005% or less, Ca: 0.0005% or more and 0.0035% or less, Ti: 0.01% or more and 0.30% or less, containing one or more lanthanoids at a total concentration of 0.0003% or more and 0.0020% or less , and the balance being Fe and unavoidable impurities The composition of each nonmetallic inclusion of 3 μm or more and 10 μm or less containing two or more kinds of lanthanoid oxysulfides, Ca oxysulfides, Al oxides, and the above The Ca oxysulfide concentration is 30% or more and 90% or less in mass%, and the mass concentration ratio of the lanthanide oxysulfide and the Al oxide (lanthanoid oxysulfide concentration)/ (Al oxide concentration) is 1.0 or more and 2.4 or less, and the number of nonmetallic inclusions satisfying the composition is 95% or more of the total number of nonmetallic inclusions. A refining method during refining, in which 35% or more and 50% or less of the total Ca addition amount is added during refining, and after 1 minute or more and 3 minutes or less, one or two selected from the group consisting of lanthanoid oxides A method for refining high-cleanliness steel, characterized by adding a flux obtained by mixing the above and the remaining Ca as Ca or a Ca alloy.

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